The invention relates to a continuously variable transmission and a working machine comprising such a continuously variable transmission.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders and articulated haulers. Although the invention will be described hereinafter with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other heavy working machines, such as dump trucks, graders, excavators or other construction equipment.
A continuously variable transmission is a transmission which speed ratio can be continuously varied over a designed range. Continuously variable transmissions are useful due to enabling operation of a prime mover at optimum speed or at optimum fuel economy while still providing a desired rotational speed at the output shaft of the transmission. In a continuously variable transmission a speed ratio between the output shaft speed to the input shaft speed of zero may be obtainable. Such a continuously variable transmission is usually referred to as an infinitely variable transmission. The existence of an output to input speed ratio of zero enables a geared neutral function where the input shaft is rotating when the output shaft stands still. The presence of a geared neutral position is useful in many industrial contexts, such as within gearboxes of working machines for example, since it may obviate the need of a starting clutch or a torque converter. Furthermore, infinitely variable transmissions facilitate separation of operation of a hydraulic system included for performing lifting operations and/or steering the working machine from the propulsion of the working machine, since the hydraulic system can be powered from a power take off at an input shaft, which may be kept running, while an output shaft connected to propulsion drive train can be kept at stand still. A typical arrangement of a continuously variable transmission includes a continuously variable unit, variator unit, having a variator input shaft and a variator output shaft connected to a planetary gear system.
An example of a continuously variable transmission is disclosed in U.S. Pat. No. 5,980,410.
U.S. Pat. No. 5,980,410, and in particular the embodiment disclosed in FIG. 2 constitutes the closest prior art and relates to a continuously variable transmission according to the preamble of claim 1.
The continuously variable transmission as disclosed in FIG. 2 of U.S. Pat. No. 5,980,410 includes a variator unit having a variator input shaft and a variator output shaft, and first, second and third planetary gear sets. The first and second planetary gear sets are arranged together and including each a first member being operatively connected to a common transmission input shaft and a second member being operatively connected to a common output shaft. Each of said first and second planetary gear sets further including a third member. A third planetary gear set having a first, second and third member is also included. The variator input shaft is operatively connected to the common output shaft and the variator output shaft is operatively connected to the third member of the first planetary gear set and selectively connectable to a transmission output shaft. The first member of said third planetary gear set is operatively connected to the third member of the first planetary gear set, and the second member of the third planetary gear set is operatively connected to the common output shaft. The third member of the third planetary gear set is selectively connectable to the transmission output shaft. Finally the third member of the second planetary gear set is selectively connectable to the transmission output shaft.
The continuously variable transmission disclosed in U.S. Pat. No. 5,980,410 enables selection between a set of operating modes having different bands of input to output speed ratios.
A problem with U.S. Pat. No. 5,980,410 is that the configuration of the continuously variable transmission will require presence of gear stages with a substantial gear ratio in order to enable the bands of input to output speed ratios to not substantially overlap. In order to provide for appropriate gear ratio of the steps additional sets of gear wheels may be needed, which makes the transmission more bulky, complex and expensive.
It is desirable to provide a continuously variable transmission which enables operation in at least three operating modes having different bands of input to output speed ratios and which transmission having a design facilitating separation of the bands of input to output speed ratios by limiting the need of additional sets of gear wheels in the transmission design.
The continuously variable transmission according to an aspect of the invention includes a variator unit having a variator input shaft and a variator output shaft, and first, second and third planetary gear sets. Each of the first and second planetary gear sets has a first member being operatively connected to a transmission input shaft which transmission input shaft is common to the first and second gear sets. Further, each of the first and second planetary gear sets has a second member being operatively connected to an output shaft which output shaft is common to the first and second gear sets. Each of said first and second planetary gear sets further including a third member.
The third planetary gear set includes a first, a second and a third member. The variator input shaft is operatively connected to the third member of said second planetary gear set and the variator output shaft is operatively connected to the third member of said first planetary gear set. The variator is thus connected in parallel to the first and second planetary gears sets.
The inclusion of the first and second planetary gear sets enables the power transmitted to be split between the variator unit and the planetary gear sets. By such a gearbox having a powersplit, in many operation modes only a minor part of the power has to be transmitted by the variator unit. The remaining power is transmitted by the planetary gear wheel unit. This implies an improved overall efficiency since the loss of energy is considerably smaller with respect to the planetary gear wheel unit compared to the variator unit. Furthermore, the size of the variator unit can be reduced.
The third planetary gear set having a first, second and third member.
The first member of said third planetary gear set is operatively connected to said variator output shaft.
The third member of said third planetary gear set is operatively connected to a transmission output shaft.
The second member of the third planetary gear set is selectively connectable to said the third member of the second planetary gear set and selectively connectable to the transmission housing. This enables a first and a second operating mode by selecting to connect the second member of the third planetary gear set to either the third member of the second planetary gear set or to the transmission housing. In the event the second member of the third planetary gear set is connected to the transmission housing, the continuously variable transmission will be worked in a first operating mode where the third planetary gear set will work as a reduction gear set enabling a large transmission ratio over the third planetary gear set. In the event the second member of the third planetary gear set is connected to the third member of the second planetary gear set, the continuously variable transmission will be worked in a second operating mode where the first, second and third planetary gear sets will work as a bridge, where the input shaft and output shaft of the variator are neither connected to the transmission input shaft nor the transmission output shaft. By the continuously variable transmission design according to an aspect of the invention, the third planetary gear set will work according to three different principles when the continuously variable transmission assumes the first, second and third operating modes. In the first mode the third planetary gear set works as a reduction gear, in the second mode the third planetary gear set merges power supplied to the first and second members of the third planetary gear set and in the third mode the thud planetary gear set is a passive unit.
Here and below the output shaft common to the first and second gear sets is referred to as the common output shaft. The common output shaft is selectively connectable to the transmission output shaft. In the event the common output shaft is connected to the transmission output shaft the continuously variable transmission will be worked in a third operating mode where power will be transmitted to the transmission output shaft from the second members of the first and second planetary gear sets. Likewise as in the second operating mode, the first and second planetary gear sets of the continuously variable transmission will in the third operating mode work as a bridge having an input shaft, an output shaft and a variator connected to two additional shafts.
The use of the third planetary gear set as a reduction gear set will enable a large gear ratio over the third planetary gear set. This facilitates separation of the bands of input to output speed ratios of the different operating modes.
The selective connection to the transmission housing may be performed by a locking mechanism arranged to lock the second member of said third planetary gear set to the transmission housing, and to release the second member of the third planetary gear set from the transmission housing, respectively.
A clutch may be arranged in between the second member of the third planetary gear set and the third member of the second planetary gear set in order to accomplish an operative connection between the third planetary gear set and the third member of the second planetary gear set or a release of the operative connection.
A clutch may be arranged in between said common output shaft and said transmission output shaft in order to allow for engagement or disengagement of the common output shaft to the transmission output shaft.
Optionally the first, second and third members of said third planetary gear set are in the mentioned order constituted by a sun gear, a ring gear and a planet carrier.
Optionally, the first, second and third members of the first planetary gear set are in the mentioned order constituted by a ring gear, a planet carrier and a sun gear, and that the first, second and third members of the second planetary gear set are in the mentioned order constituted by a planet carrier, a ring gear and a sun gear.
Here and below, a planet carrier common to the first and second planet gears is referred to as a common planet carrier and a ring gear common to the first and second planetary gears are referred to as a common ring gear. Optionally the first members of said first and second planetary gear sets are constituted by a common planet carrier and that said second members of said first and second planetary gear sets are constituted by a common ring gear and that said third members of said first and second planetary gear sets are constituted by a first and a second sun gear.
Optionally, the first members of said first and second planetary gear sets are constituted by a common ring gear and that said second members of said first and second planetary gear sets are constituted by a common planet carrier and that said third members of said first and second planetary gear sets are constituted by a first and a second sun gear.
Here and below a transmission input shaft common to the first and second planetary gear sets, that is engaging, with or otherwise being connected to the first members of both the first and second planetary gear sets, are referred to as a common transmission input shaft. A mode selection arrangement may be provided which enables different operating modes with different bands of continuously variable speed ratios between said common transmission input shaft and said transmission output shaft.
A total speed ratio itotal may be defined as the rotational speed of the transmission output shaft ωtransmission, output to the rotational speed of the transmission input shaft, that is:ωtransmission, input, itotal=ωtransmission, output/ωtransmission, input.
A variator speed ratio may be defined as the rotational speed of the variator output shaft ωvariator, output to the rotational speed of the variator input shaft ωvariator, input, that is;ivariator=ωvariator, output/ωvariator, input.
Preferably a first hand of the first operating, mode extends from a total speed ratio itotai over the continuously variable transmission, which is equal to zero for a variator speed ratio of zero, to a ratio itotal=k for a variator speed ratio of infinity. A second band of a second operating mode preferably extends from ratio itotal=k for a variator speed ratio of infinity, to a ratio itotal=m for a variator speed ratio of zero. A third band of a third operating mode preferably extends from ratio itotal=m for a variator speed ratio of zero to a ratio itotal=n for a variator speed ratio of infinity. In order to enable the third band to meet the second band at a variator speed ratio of zero, a gear stage may be arranged in between the common output shaft and the transmission output shaft.
The mode selection arrangement may include a locking mechanism, a first clutch, a second clutch and a set of actuators. The locking mechanism is arranged to selectively lock and release said second member of said third planetary gear set to the transmission housing. The first clutch is arranged in between said second member of said third planetary gear set and said third member of said second planetary gear set. The second clutch is arranged in between said common output shaft and said transmission output shaft.
Further a set of actuators are arranged to control engagement and disengagement of said first and second clutch and arranged to control release and locking of said locking mechanism.
The mode selection arrangement may include a controller arranged for operating the set of actuators controlling the operation of the locking, mechanism, first clutch and second clutch.
Mode changes are preferably performed when the rotational speeds at input and output of the clutch next in turn to be engaged are synchronous.
The controller enables selective operation of the continuously variable transmission in a first operating mode where the locking mechanism is locked and said first and second clutches are disengaged, in a second operating mode where the locking mechanism is released, said first clutch is engaged and the second clutch is disengaged, and in a third operating mode where the locking mechanism is released, the first clutch is disengaged and the second clutch is engaged.
Optionally the variator unit is of hydraulic type. In this case the variator unit includes a first hydraulic machine provided with the input shaft of the variator unit and a second hydraulic machine provided with the output shaft of the variator unit, the first and second hydraulic machines being hydraulically connected to each other.
The configuration of the continuously variable transmission allows the variator unit to be controlled by a dual yoke enabling simultaneous control of displacements of the first and second hydraulic machines via a single actuator. The continuously variable transmission is enabling at least three different operating modes having a ratio ivanator=wVanator, output/wVanator, input with the same sign for each operating mode thereby enabling, common control for the first and second hydraulic machines via a single actuator.
Optionally the variator unit is of an electric type. In this case the variator unit includes a first electric machine provided with said input shaft of the variator unit and a second electric machine provided with the output shaft of the variator unit, the first and second electric machines being electrically connected to each other.
The invention, according to an aspect thereof, may be particularly useful in connection with a working machine, such as a wheel loader.
A working machine may be provided with a bucket, container or other type of implement for digging, lifting, carrying and/or transporting a load. A working machine may be operated with large and heavy loads in areas where there are no roads, for example for transports in connection with road or tunnel building, sand pits, mines and similar environments.
A working machine, such as a wheel loader is usually provided with a prime mover, such as an internal combustion engine, a transmission line with a torque converter and a gearbox for supplying power to the driving wheels of the wheel loader. In addition to supply power to the driving wheels, the internal combustion engine has to supply power to one or more hydraulic pumps of a hydraulic system of the wheel loader. Such a hydraulic system is used for lifting operations and/or steering the working machine.
Hydraulic working, cylinders may be arranged for lifting and lowering a lifting arm unit, on which a bucket, forks or other type of attachment or working tool is mounted. By use of another hydraulic working cylinder, the bucket can also be tilted or pivoted. Further hydraulic cylinders known as steering cylinders are arranged to turn the wheel loader by means of relative movement of a front body pan and a rear body part of the wheel loader which body parts are pivotally connected relative to each other.
The gearbox is a continuously variable transmission as described above which gives a lot of advantages compared to a stepped gearbox. For example, the velocity of the working machine can be controlled independently of the speed of rotation of the prime mover. If the continuously variable transmission has a speed of rotation range comprising a mode where the rotation speed of the output shaft of the gearbox is zero or close to zero independently of the rotation speed of the input shaft of the gearbox at the same time as torque can be transmitted from the input shaft to the output shaft (usually referred to as an infinitely variable transmission (IVT) with geared neutral), the torque converter traditionally used in working machines can be omitted. By the expression “zero or close to zero” is meant a speed of rotation of the output shaft which is zero or in the size of a few rotations per minute.
By the continuously variable transmission and the geared neutral function the operation of the hydraulic system can be separated from the propulsion of the working machine for all vehicle speeds. Furthermore, the hydraulic system can be driven by the prime mover when the working machine stands still without using any clutch for disengagement of the prime mover relative to the transmission line.